University of Illinois researchers have developed a cloak that can be used underwater to hide objects from sonar and other ultrasound waves.

Nicholas Fang, study leader and mechanical science and engineering professor at the University of Illinois, along with fellow researchers at the university, have created an acoustic, underwater cloak as a way of controlling sound waves.

Up until this point, scientists have been trying to push the use of materials that wrap sound around objects instead of absorbing or reflecting it. But turning the concept into a reality has been difficult.

Now, Fang and his team have made a prototype of a cloak that can make an object seem invisible to a variety of different sound waves.

"We are not talking about science fiction," said Fang. "We are talking about controlling sound waves by bending and twisting them in a designer space. This is certainly not some trick Harry Potter is playing with."

To make this cloak, researchers used metamaterial, which is a class of artificial materials that are engineered to have altered properties. The cloak is two-dimensional and cylindrical, and has 16 concentric rings of acoustic circuits, where each ring has a different index of refraction. This allows the cloak to help guide sonar and other ultrasound waves.

"Basically what you are looking at is an array of cavities that are connected by channels," said Fang. "The sound is going to propagate inside those channels, and the cavities are designed to slow the waves down. As you go further inside the rings, sound waves gain faster and faster speed."

Researchers tested the cloak on a steel cylinder, among other objects with different shapes and sizes. The cloaked object was submerged underwater, and when sound wavelengths were transmitted, the sound waves would propagate around the cloak's outer rings and the acoustic circuits actually bent the waves so that they'd wrap around the outside of the cloak. Tests showed that cloak provided "acoustic invisibility" to sound waves from 40 to 80 KHz.

"This is not just a single wavelength effect," said Fang. "You don't have an invisible cloak that's showing up just by switching the frequencies slightly. The geometry is not theoretically scaled with wavelengths. The nice thing about the circuit element approach is that you can scale the channels down while maintaining the same wave propagation technology."

Fang and his group would like to see this cloak used as an aid for military stealth, health care and soundproofing, but more research is needed to iron out a few kinks in the technology, such as formation and implosion of bubbles, called cavitation, in fast-moving underwater objects.